1. Field of the Invention
This invention relates generally to a process for removing the residual tensile welding stresses in the inner layer of a weld metal and heat affected zone that joins two dissimilar metals and more particularly for removing tensile residual stresses in a circumferential weld that joins a pressurized water reactor pressure vessel nozzle and a safe-end that are butt-welded to each other end to end.
2. Description of the Prior Art
When piping is butt-welded together by means of a circumferential weld, significant residual tensile welding stresses are produced in the weld metal and in the heat-affected zone of the piping. These tensile stresses tend to enhance stress corrosion cracking in the weld region and the resulting crack propagation in the weld metal and in the heat-affected zone of such piping.
Stress corrosion cracking in stainless steal piping has been a serious drawback in boiling water reactor (BWR) plants in the United States and elsewhere in the world. Mitigating remedies have included hydrogen addition to the water flowing through the pipes, the use of the improved welding techniques and the use of better materials in the preparation of the steel piping. However, the occurrence of cracks have not been fully eliminated in the BWR plants.
Induction heating stress improvement (IHSI) is one method currently being used to improve residual welding stresses in piping welds. IHSI is a thermal process requiring precise time and temperature controls. While suitable for simple pipe to pipe welds it has potential concerns of successful applications to multi-metal nozzle to safe-end welds with complex configurations.
O'Donnell et al. U.S. Pat. No. 4,612,071 describes a mechanical stress improvement process for reducing the tensile residual welding stresses in the weld metal, heat-affected zone and in the adjacent base metal of stainless steel piping for BWRs where the piping is butt-welded together end to end with a circumferential weld. The process introduces compressive stresses in the weld metal, heat-affected zone and adjacent base metal by permanently reducing the diameter of the adjacent pipe(s).
The permanent reduction in diameter required by the O'Donell et al. process has to be at least 0.2% but less than 1% and preferably in the range of about 0.2% to about 0.8%. The compressive stresses were imposed on the weld metal, heat-affected zone and on the adjacent base metal using split steel rings that were forced together by peripheral flange bolts.
U.S. Pat. No. 4, 683,014 describes an improvement in the mechanical stress improvement process wherein the radial load is applied inwardly on a section of at least one of the piping elements away from the weld such that the distance from the midplane of the section of the piping element upon which the radial load is applied to the weld midplane is equal to about 2 to about 12 times the thickness of the piping element upon which the radial load is applied. The distance from the edge of the section of the piping element upon which the radial load is applied that is adjacent to the weld to the weld midplane is at least equal to about one-half of the thickness of the piping element upon which the load is applied. The amount of radial load applied is sufficient to permanently reduce the outside diameter at the midplane of this section of the piping element in the range of about 0.2% to about 2.0%.
Stress corrosion cracking has more recently been observed in the nozzle ends of pressurized water reactor nuclear steam supply system primary components, such as the reactor vessel inlet and outlet nozzles and the pressurizer surge line, spray, safety and relief nozzles. While a significant cause of the weld cracks in pressurized water reactors may be similar to that identified in BWRs i.e. post weld residual tensile stresses, the geometry of the nozzles and safe-ends are much different than are found in boiling water reactors. The nozzles on the boiling and pressurized water reactor pressure vessels are forged from low alloy carbon steel and are connected to the stainless steel coolant piping through a high nickel-chromium alloy intermediate coupling known as a safe-end. The pressure vessel nozzle and the safe-end are butt-welded together, end to end, with a high nickel-chromium alloy weld material. The thicknesses of the PWR safe-ends are much greater and their lengths generally shorter than the BWR safe-ends. The geometry of the nozzles and the short lengths of the thicker safe-ends do not readily lend themselves to the mechanical stress improvement process taught by either U.S. Pat. Nos. 4,683,014 and 4,612,071 for relieving the stresses on the nozzle to safe-end weld.
Application of structural weld overlay reinforcement on the external surface of the nozzle-end and weldment is being considered as a remedy against initiated or potential primary water stress corrosion cracking. However, the process would have a high cost and radiation exposure and require an extended outage time.
Removal of an inside layer of the susceptible weld material and deposition of a less susceptible high nickel chromium alloy weld metal is offered either as a repair of existing cracks or as a preventative measure to mitigate primary water stress corrosion cracking potential. This process requires access to the inside of the pressure vessel nozzle in a dry condition. This repair process will also have a high cost and radiation exposure and requires an extended outage time.
Shot peening is a process that produces a thin surface layer where the material is under compression. The presence of the surface compression would prevent the initiation of primary water stress corrosion cracking. It is evident however, that any such induced compression would not stop the propagation of existing cracks that extend beyond the depth of the compressive layer. Also the long-term behavior of the thin compressive layer following operational thermal cycles is not known.
Accordingly, a new weld repair process is desired that can be applied to relieve the stresses in the nozzle to safe-end welds in a pressurized water reactor steam supply system.
Furthermore, such a process is desired that will relieve the tensile stresses on the inner surface of such welds and permanently introduce compressive forces in such welds, which extend outward from the inner weld surface.
Additionally, such a process is desired that can be performed relatively quickly, during a plant outage to minimize radiation exposure.